Toothbrushes are well-known articles and normally comprise a head and a grip handle, generally with a narrowed neck region in between, all arranged along a longitudinal head-handle direction. Normally toothbrushes are made primarily of a plastic material, e.g. a polypropylene or styrol acryl nitrol (“SAN”). Many are so called two-component toothbrushes comprising a part made of plastic, e.g. the head and the main structure or “skeleton” of the handle, and a part made of a softer resilient elastomer, such as a thermoplastic elastomer e.g. Santoprene™ e.g. comprising part of the grip handle, to enhance grip, aesthetic appearance or to influence flexibility. Such toothbrushes are disclosed for example in U.S. Pat. No. 5,054,154 and EP-A-0 336 641 among others. Toothbrushes are also known incorporating one or more elastomer rib on the back of the brush head (opposite surface to that on which are mounted the bristles)”, e.g. WO-A-04/041025, for tongue cleaning. A toothbrush of this type is sold commercially by the present applicant under the name Aquafresh extreme clean™.
Toothbrushes are normally made by a process in which the plastic material part of their structure is first made, generally by injection moulding. Then this plastic part is enclosed in a mould cavity which defines the shape of the elastomer material part, and the elastomer material of the second component in a fluid state is injected into this mould cavity to form the elastomer material part. Normally the plastic material is formed with one or more cavity to receive this second material. U.S. Pat. No. 3,633,237 discloses such a toothbrush head having cavities in the plastic material part with which the elastomer part can engage.
Normally such injection moulds are mould assemblies comprising one or more, generally two, mould blocks each with a part mould cavity therein, which when brought together in the assembled mould mate so that their part mould cavities combine to form a complete mould cavity between them, the surface across which they mate being known in the art as the “split line”, and termed herein the “primary split line”. Such processes are well known. WO-A-02/13656 for example describes such a process and suitable moulding machinery for its performance being a so called stack mould. It is also known for injection moulds to incorporate vents for the escape of air or other gases within the mould cavity, see for example DE-A-101 04 034, JP-A-2122910, JP-A-60046222, and U.S. Pat. No. 3,555,619.
There is a problem in making toothbrush heads having an elastomer material part in the form of elastomer ribs extending transverse to the longitudinal direction, because during the injection of the elastomer material into the mould cavity air can become trapped in the parts of the mould cavity which define these ribs, for example as fluid elastomer material flows simultaneously from transversely opposite ends of these parts to trap air between these ends. This can result in voids in the so-formed elastomer part. It is an object of this invention to address this problem by providing a process and mould in which inter alia venting of air is improved, and flow of fluid elastomer material in the mould cavity is improved.